ISSN 2029-7106 print / ISSN 2029-7092 online
ISBN 978-9955-28-826-8 (1 Volume)
ISBN 978-9955-28-827-5 (3 Volumes)
ENVIRONMENTAL ENGINEERING
th
The 8 International Conference
May 19–20, 2011, Vilnius, Lithuania
Selected papers
http://enviro.vgtu.lt
© Vilnius Gediminas Technical University, 2011
METHOD OF REMOVAL OF THE PLASTIC INSULATOR FROM WASTE CABLES BY
PASSING THEM BETWEEN TWO ROTATING ROLLS THAT HAVE DIFFERENT
SURFACE TEMPERATURES
Vadim Moksin1, Vytautas Striska2, Ina Tetsman3
1
Vilnius Gediminas technical university, J. Basanavičiaus str. 28, LT-03224 Vilnius, Lithuania.
E-mails: [email protected]; [email protected]; [email protected]
Abstract. Recycling of cable waste, especially the complete separation of metal and insulating material for reuse has
always been a problem. Especially this problem is relevant for small enterprises because present processes to recover
insulating material consists of many stages and requires expensive equipment. The processes usually involve multistage grinding of the cables into smaller pieces and separating them into metal and plastic fractions.
The problem can be solved by thermal methods based on the differences in coefficient of thermal expansion of the
insulating material and conductor. By one of these methods the cable is passed between two rotating rolls, while surface of the one roll is heated below the melting temperature of thermoplastic insulator, surface of next roll stays cold.
Thus the thermoplastic insulator of the cable breaks after the total impact of thermal deformation of insulator and
metal conductor and softening of material of the insulator. Conductor remains undamaged and can be re-covered by
new plastic insulating material layer. The experimental cable processing machine was designed and produced in the
Laboratory of machines and technologies of the Vilnius Gediminas technical university, initial tests were performed.
Such machine can be used by small cable manufacturing and waste management companies, electric devices repair
and manufacturing workshops. Results of experimental research are presented as optimal peripheral speed of the rolls
(or cable speed) versus diameter of the copper and aluminum conductor of the cable, when the temperature of the
“hot” roll equals 160°C.
Keywords: waste cable, conductor, copper, aluminum, PVC insulator, roll, temperature.
1. Introduction
It is known that European waste management hierarchy is presented following (Staniškis et al. 2004; Osibanjo and Nnorom 2007):
− prevention or waste minimization
− materials recycling and reuse
− incineration with energy recovery
− incineration
− landfilling
It should be noticed that waste minimization to zero
can’t be achieved for many manufacturing processes at present, and only second principle in waste management hierarchy can be realized in many companies. It is not very difficult to recycle clean and homogeneous waste, but there can
be serious problems with composite products made of different materials (for example, plastic mixed with metals,
rubber, paper, other kinds of plastics, etc). In such a case, the
waste separation (mostly multistage and high-priced) must
be included in waste recovery process.
One such composite product is waste cables. The cable scrap can be produced during manufacturing process
(for example, rejected for insulating material defects) and
due to the end of life of cables (for example, during repair
of electric devices and buildings). The most valuable component of the cable that must be recycled is non-ferrous
metals (copper, aluminum). Next, thermoplastic insulator
(like polyvinylchloride, polypropylene, and polyethylene)
can be successfully granulated and reused in plastic molding machines. It is established that plastic insulated cable
scrap contains 40–90 wt% of metals (Hitoshi 2000).
The main cable waste recovery problem is how to remove plastic insulator from metal conductor. In the past
cables were recycled by simply burning them. The copper
remained solid and could be collected after burning. Although burning cables was a simple and efficient technology, such thermal recycling is no longer allowed in many
countries owing the release of heavy metals, dust, and harmful gases (hydrogen chloride, dioxins, etc) into the environment. In addition, it is not economic to recover only the metals, without considering the insulating material.
228
At present mechanical separation technologies are
used. In general two technologies are being used to recycle cable scrap (Sijstermans 1997; Al-Salem et al. 2009):
− (cryogenic) shredding of cables
− stripping of cables
The room temperature shredding process involves a
multistage grinding of cables into smaller pieces, separating
them into metal and plastic fractions. The mostly used cable
recovery equipment includes in that case the shredders,
sieves, and various separators (for example, air table, electrostatic separator, floatex® density separator) used to separate metal from plastics and other materials (Tilmantine et
al. 2009; Nemerow et al. 2009; Corbitt 1999).
In the cryogenic shredding process, cables are treated
with liquid nitrogen to make the insulating material more
brittle. After liberation, the metal particles are separated
from the insulating particles by using the differences in density, conductive and magnetic properties. The cryogenic
process did not find wider application due to the high cost of
liquid nitrogen and cryogenic equipment (Sijstermans 1997).
Shredding process uses an excessive amount of energy
for multistage grinding the cables into the smaller pieces.
The shredding technology has a high capacity, so, it is
mostly used in the big waste recycling companies. It is also
is flexible, because more types of cable can be processed.
Shredding process results in some impregnation of
the insulating material with fine metal particles. Impregnated metals are difficult to separate from insulating material, it is important for the reuse of insulating material to
be absolutely metal-free.
In the stripping process the insulating material is being
split and peeled of the conductor. By using stripping technology to process cable scrap, higher metal grades and recoveries can be achieved. The stripping machine is often
manually fed, because it is necessary to straighten and precisely orient the cable in respect of the cutting rolls. Therefore stripping method has a low capacity. Cables with a
small diameter cannot be stripped and must be shredded.
The insulator can be also removed from cable manually, but this process is not productive. Next, the insulator
can be removed from conductor chemically with the help
of solvents (Corbitt 1999), but in such a case solvents
must be cleaned and utilized.
Further, the thermal methods can be used to remove
the insulator from conductor. These methods are based on
the differences in coefficient of thermal expansion of the
insulation material and metals, with no need to grind the
cable into small pieces to improve the liberation of metal.
The cables can be cut into a suitable length and placed
into a blender with hot water (Shein and Tsai 2000). By
controlling the water temperature, blending speed and
cutting length, complete separation can be achieved.
The thermal method has been proposed with no need to
cut cables into the pieces. The single-core waste cable is
placed between two rotating rolls (Fig. 1). Surface of the one
roll is heated below the melting temperature of thermoplastic
insulator and surface of second roll remains cold. The preparation of the cables consists only of rough straightening of
the cables, method can be easily automated. It means that
cable feeding and guiding device can be also used to avoid
cutting the long cables into pieces. Different diameter, length
and material single-core cables can be processed by this
method. The metal conductor remains undamaged and free
from insulating material; it can be re-covered by plastic insulator during further operations.
N
v
3
4
v
2
5
v
Fig 1. Scheme of removing the insulator from metal conductor: 1 – cable; 2 – “cold” roll; 3 – “hot” roll; 4 – metal
core (conductor); 5 – plastic insulator; v – peripheral speed
of the rolls and linear speed of the cable; N – load
Fig 2. Experimental stand: 1 – “cold” roll; 2 – “hot” roll; 3 – current collector; 4 – reduction gear; 5 – electromotor
229
1
Table. Properties and composition of tested cables
Diameter of
the cable, mm
3
4.4
6.8
Diameter of the
conductor, mm
1.85
2.8
4.6
Thickness of insulating
material (PVC) layer, mm
0.575
0.8
1.1
Metal, wt%
Al core
Cu core
58
82
60.5
83.5
65.5
86.2
PVC, wt%
Al core
Cu core
42
18
39.5
16.5
34.4
13.8
2. Experimental stand
4. Results and discussion
The photo of experimental stand is presented in Fig.
2. Electromotor 5 rotates rolls 1 and 2 via belt transmission and reduction gear 4. Roll 1 rotates at the same
speed as roll 2. Both the rolls have equal diameters 89
mm. Roll‘s 2 surface is heated below the insulating material melting temperature by means of current collector 3.
Temperature of the surface of “hot” roll can be varied
from room temperature to 250ºC. It is controlled by IR
thermometer. The rolls are pressed one to another by
pneumatic cylinder located inside the reduction gear 4.
The rotational speed of the rollers (or cable speed) is varied by means of variable-frequency drive (not shown in
Fig. 2) connected to electromotor 5. Both rolls made from
special manganese stainless steel to reduce adhesion of
insulating material to the surfaces of the rolls. Rotational
speed of the rolls is measured by means of stroboscope
(not shown in Fig. 2).
The results of experiments are presented in Fig. 3 as
average optimal peripheral speed of the rolls (or cable
linear speed) versus diameter of the copper conductor of
the single-core cable.
Fig. 3 illustrates that when the diameter of copper
conductor of the cable increases from 1.85 to 4.6 mm
optimal peripheral speed of the rolls required for complete removal of insulator decreases from 1.8 to 0.41
m/min (rotational speed of the rolls decreases from 6.5 to
1.5 rpm). When the diameter of aluminum conductor increases from 1.85 to 4.6 mm peripheral speed of the rolls
decreases from 1.64 to 0.29 m/min (rotational speed of
the rolls decreases from 5,9 to 0,9 rpm).
It is established that results of experiments can be
approximated with following power functions:
3. Object of research and experimental technique
Thr = Tm − (50 − 60)
(1)
where Thr is surface temperature of “hot” roll, ºC, Tm is
melting temperature of cable insulating material, ºC (212ºC
for PVC insulator (Макаров and Коптенармусов 2003)).
Before the experiments all tested cables were cut up
into 2 m length pieces. These cable pieces were later placed
between rotating rolls of experimental stand (Fig. 2). Quality
of removal was controlled visually. If the incomplete separation of the insulating material was observed, the rotational
speed of the rolls was reduced and additional test was performed. This procedure was repeated as long as complete
removal of the insulator was observed, then the last rotational speed of the rolls was accepted as optimal speed for
the given diameter of conductor of the cable. Next, the
thicker cable was tested. Every test at optimal speed of the
rolls was performed five times in order to minimize the influence of the random errors on results.
(2)
v = 5.27d -1.9
(3)
where v is optimal peripheral speed of the rolls, m/min, d
is the diameter of the conductor (core) of the cable, mm.
Equation (2) is valid for copper conductor cables and
equation (3) is valid for aluminum conductor cables. Equations closely corresponds to results of experiments, theirs
coefficients of determination 0.99 and 1 respectively.
2
Optimal peripheral speed, m/min
Copper and aluminium single-core cables covered by
single layer of polyvinylchloride (PVC) insulator were approved as object of experimental investigation. Parameters
of the cables are presented in Table. Surface temperature of
the “hot” roll during experiments was 160ºC while surface
temperature of the “cold” roll equalled to room temperature
(approx. 20ºC). Surface temperature of “hot” roll must be
less than melting point of the material of insulator. It also
must be high enough to soften the insulating material. It is
recommended (Striška and Mokšin 2008) to calculate it by
following formula:
v = 4.71d -1.62
1,8
1,6
1,4
1,2
Cu wire
1
Al wire
0,8
0,6
0,4
0,2
1,5
2,5
3,5
4,5
Diameter of the conductor, mm
Fig 3. Optimal peripheral speed of the rolls as function of
diameter of the conductor of single-core cable
As is evident from Fig.3, the insulating material can be
removed from copper conductor cables at higher peripheral
speed of the rolls as compared with aluminum conductor
230
cables. When the 1.85 mm copper conductor cables are used
the optimal speed of the cable increases 1.1 times as compared with aluminum conductor cables of the same conductor diameter. When 2.8 mm copper conductor cables are
processed speed increases 1.1 times. 6.8 mm copper conductor cables show a 1.4-fold increase in speed.
The results can be explained by differences in coefficient of thermal expansion and thermal conductivity of
the insulating material and conductor. When the one side
of the cable is heated, the hot section at the contact place
is lengthened according to well-known law:
∆L = α L L ∆T
ger thermal conductivity, thus conductor is lengthened
evenly and breaks through the soft PVC layer. Aluminum
conductor diverges from vertical more than copper conductor due to less thermal conductivity and larger thermal expansion coefficient, the optimal speed is decreased.
The productivity diagram is presented in Fig. 5. Evaluating the density of conductor material, it can be stated that
machine can liberate 0.0239 kg copper and 0.0073 kg aluminum per hour, while diameter of conductor of the cable
equal to 1.85 mm. For 2.8 mm conductor cables these values
are 0.0548 kg/h and 0.0166 kg/h respectively. For 4.6 mm
conductor cables – 0.148 kg/h and 0.045 kg/h.
(4)
120
where ∆L is change in linear dimension, m, αL is linear
thermal expansion coefficient, 1/ºC, L is initial length, m,
∆T is change of temperature, ºC.
The linear thermal expansion coefficient of PVC (insulating material) is significantly larger than coefficients of
copper or aluminum (52·10–6 1/ºC (PVC, at 20ºC),
23·10–6 1/ºC (Al, at 20ºC), 17·10–6 1/ºC (Cu, at 20ºC)). This
means that length of PVC section in the contact with hot roll
zone increases more than length of conductor, which has
lower temperature (Fig. 4) due to much less thermal conductivity of PVC layer. The difference between temperatures of
the insulator near “hot” and “cold” rolls is significant (Fig.
4), section of PVC insulator near the “hot” roll has an increased temperature, opposite side remains colder, and the
insulator starts to diverge from vertical. The temperature is
more evenly distributed in the conductor (Fig. 4) due to big-
104,12
100
98.13
80
Productyvity, m/h
60
53,15
44,62
Cu wire
40
Al wire
23,75
20
17,36
0
1,85
2,8
4,6
Diameter of the conductor, mm
Fig 5. Productivity of removal of the insulator from the cables
as function of diameter of conductor of single-core cable
Fig 4. Temperature distribution in single-core cable (cross-section is shown) calculated with SolidWorks© Simulation software
(material of conductor – copper, material of insulator – PVC, diameter of conductor – 4.6 mm, diameter of the cable – 6.8 mm)
231
5. Conclusions
References
1. Proposed two rolls insulator removal machine can be
used by small cable production or waste recycling
companies. One of the main advantages of machine is
the possibility to keep the metal conductor of the cable
undamaged and eliminate preparatory operations with
the waste cables. The undamaged conductor can be recovered by plastic in further stages.
Al-Salem, S. M.; Lettieri, P.; Baeyens, J. 2009. Recycling and
recovery routes of plastic solid waste (PSW): a review.
Waste Management, 29(10): 2625–2643.
Corbitt, R. A. 1999. Standard handbook of environmental engineering. New York: McGraw-Hill. 1005 p. ISBN
0070131600.
Hitoshi, S.; Koji, O.; Kazumi, I.; Fumio, A. 2000. The consideration of cable recycle. Showa Electric Wire and Cable
Review, 50(1): 39–43.
Nemerow, N. L.; Agardy, F. J.; Sullivan, P.; Salvato, J. A. 2009.
Environmental engineering – Environmental health and
safety for municipal infrastructure, land use and planning,
and industry. 6th ed. Hoboken: John Wiley&Sons. 556.
ISBN 978-0-470-08305-5.
Osibanjo, O.; Nnorom, I. C. 2007. The challenge of electronic
waste (e-waste) management in developing countries.
Waste Management and Research, 25(6): 489–501.
Shein, S.; Tsai, M. 2000. Hot water separation process for copper and insulating material recovery from electric cable
waste. Waste Management and Research, 18(5): 478–484.
Sijstermans, L. F. 1997. Recycling of cable waste. In Proc. of 14th
International Conference and Exhibition on Electricity Distribution (CIRED’1997), Birmingham, UK, 1997, 53–58.
Staniškis, J. K.; Bagdonas, A.; Silvestravičiūt÷, I.; Uselyt÷, R.;
Česnaitis, R.; Karaliūnait÷, I.; Miliūt÷, J.; Šlenorait÷Budrien÷, L.; Varžinskas, V. 2004. Integruota atliekų
vadyba [Integrated waste management]. Kaunas:
Technologija. 368 p. ISBN 9955-09-751-5.
Striška, V., Mokšin, V. 2008. Research into efficiency of plastic
insulator removal from single-core cables. Aplinkos tyrimai, inžinerija ir vadyba [Environmental Research, Engineering and Management], 1: 43–47.
Tilmantine, A.; Medles, K.; Bendimerad, S.-H.; Boukholda, F.;
Dascalescu, L. 2009. Electrostatic separators of particles:
application to plastic/metal, metal/metal and plastic/plastic
mixtures. Waste Management, 29(1): 228–232.
Макаров, В. Г.; Коптенармусов, В. Б. 2003. Промышленные
термопласты [Industrial thermoplastic materials]. Москва: АНО Химия. 208 с. ISBN 5-98109-001-4.
2. The recovered insulating material after the separation
process is absolutely metal-free and can be reused
without additional operations.
3. The optimal peripheral speed of the rolls (or linear cable speed) required to completely liberate polyvinylchloride insulating material from conductor of singlecore cable depends on the diameter of the conductor.
When diameter of copper conductor increases from 1.8
to 4.6 mm, optimal peripheral speed of the rolls must
be reduced from 1.8 to 0.41 m/min (rotational speed of
the rolls must be reduced from 6.5 to 1.5 rpm). When
diameter of aluminum conductor increases from 1.8 to
4.6 mm, optimal peripheral speed of the rolls must be
reduced from 1.64 to 0.29 m/min (rotational speed of
the rolls must be reduced from 5.9 to 0.9 rpm).
4. The optimal peripheral speed of the rolls (or linear cable speed) required to completely liberate polyvinylchloride insulating material from conductor depends on
the material of conductor of single-core cable. Insulating material can be removed from copper conductor
cables at higher peripheral speed of the rolls as compared with aluminum conductor cables (1.1–1.4 times,
depending on the diameter of conductor). This phenomenon can be explained by differences in thermal
conductivities of the materials.
232